Tesla is a SI (International System) unit of measurement for magnetic field strength and was named after the Serbian born scientist Nikola Tesla. One Tesla is equal to 10,000 gauss. The earth’s magnetic field is about 0.5
Super conducting MRI scanners use Tesla to measure their magnetic force
To describe the “magnetic force” in a typical scanner MRI technologists will refer to the MRI unit by brand name and Magnetic field strength in Tesla. For example: GE 3.0 T. Most hospitals and private practices, use 1.5T and 3.0T magnets. Newer models, 7.0T and above are being developed for medical imaging centers throughout the world.
Why is magnetic strength important when considering an MRI machine?
The higher the magnetic strength the better the image quality which gets down to (SNR) Signal to Noise Ratio.
SNR can be defined as the difference in signal intensity received from the surface coil that is on the body part being imaged and the surrounding air or background noise. The more intense the signal and the less the background noise, the higher the SNR.
The higher the signal to noise ratio the more accurate the images. For example, in a perfect system, a 3.0T unit vs a 1.5T scanner the signal to noise ratio will be doubled. However, the improvement is only 30-60% and not 100% due to MRI artifacts. These artifacts come from prosthetic devices, inhomogeneities of the magnetic field and dental work.
Why do higher magnetic field strengths produce quality images?
The simple answer is higher magnetic field strengths increase the signal in the cells and decreases the noise which distorts the image. The strong magnetic field can penetrate human tissue without being hindered by the thicker parts of the anatomy. The stronger magnetic field will decrease scan times and provide superior image quality without the use of contrast.
How do strong magnetic fields, water and radio frequencies produce images? Here’s the skinny on MRI physics
The Human body is made up of 65% water and MRI scanners use the water in body cells (hydrogen atoms or protons in that water) to produce a signal for imaging. Due to cellular breakdown, damaged or diseased tissues have more water content in their cells than healthy tissue.
Water is a polar molecule which means it has a positive and negative end. Polar molecules wobble and rotate in random directions. The motion is similar to a toy top. Angular momentum and gravity produce the characteristic wobble. However, when placed in a magnetic field the polar molecules align with the magnetic flux lines inside the scanner. They no longer wobble in random directions.
Once lined up, a radio frequency is pulsated through the proton or polar molecule. The proton is knocked either 90 or 180 degrees against the magnetic flux lines. When the RF frequency is turned off the proton returns to its original position giving off a signal. This signal is read by the RF coil inserted around the body part being imaged. The various chemicals in human tissue give off different signals which produces an image of the body part.
For example, if there is tissue damage, the protons, or polar molecules, will give off a different signal versus normal tissue. Fat molecules produce a lighter image than water molecules. Damaged tissue produces a darker image on the scan.
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Tesla is a unit of measure for magnetic field strengths in MRI units
Tesla is simply a unit, like inch, centimeter, F and C and was named after the Serbian born scientist Nikola Tesla. The unit is used to indicate the strength of a Super Conducting MRI Scanner.
Most medical practices use field strengths of 1.5T to 3.0T for their MRI units and some centers are utilizing of 7.0T and above. However, Tesla strengths higher than 1.5T provide better images without contrast but require stricter safety protocols to be followed. See more about how to prevent MRI accidents in this other article.